The invention relates to a method according to a method for determining positions of a bar code on a support relative to a sensor array and an apparatus for determining positions.
In many applications, a support having at least one bar code trace and a sensor arrangement for registering at least one piece of bar code information are used for determining relative positions between two elements. The registered bar code information permits the determination of the sensor position relative to the bar code or to the support having the bar code. In the determination of the linear positions, linear bar codes or graduations are used. In the determination of angular positions, bar codes in the form of circular graduations are used.
For position determination, there are firstly solutions which directly determine the absolute position and secondly those which determine a starting position and incremental information. The bar code traces are adapted to the respective solution.
Position determinations with incremental information must start from an exactly detectable zero point and exactly detect movements in both directions. The direction of movement is determined, for example, via the phase position between the signals of two sensors arranged offset along the bar code. For high accuracy, extremely small bar spacings, a plurality of traces side by side and/or interpolation methods have to be used.
In the determination of an absolute position, codes on a plurality of traces, codes on one trace, readings with a sensor array, for example a diode array, or dynamic readings, for example with a rotating circular graduation, can be used.
An angular position can be exactly determined, for example, by means of a circular graduation and a sensor array extending over a section of the circular graduation, in particular a diode array. For this purpose, the circular graduation is divided, for example, into absolutely coded intervals, the sensor array reading the interval number and deriving a coarse angle determination therefrom. In order to obtain an exact angle value, the position of an interval mark relative to the sensor array is determined from the intensity values of the sensor array.
The documents EP 085 951 B1 and DE 199 39 643 A1 describe various solutions for determining the position of the interval mark. According to EP 085 951 B1, the centre of gravity of the interval mark is determined. In DE 199 39 643 A1, the position of the interval mark is determined by detecting the flank positions of many graduation lines and determining a single precise value therefrom.
For exact absolute position determinations, evaluation times which are too long in the case of various applications are required.
DE 35 28 955 A1 discloses an angle measurement for geodetic devices, in which a rotating graduated circle comprises both an absolutely coded graduation and an incremental graduation. The magnitude of the angle is determined from readings of the two graduations. The effort for the two graduations and the two readings is too great.
DE 41 25 865 A1 discloses a solution in which an absolute position determination is also permitted with a plurality of incremental graduations which run side by side and have different pitches.
DE 44 36 784 A1 discloses a solution in which an incremental line graduation, i.e. a trace, simultaneously comprises a code which permits an absolute position determination. The code is formed by virtue of the fact that the lines of the graduation have different widths but their centre lines have constant spacings. This absolute code is read by a CCD array. An incremental read head permits the incremental position determination. The absolute and incremental position determination can be tested mutually for plausibility. An eccentric error can also be eliminated. If appropriate, the absolute position determination is carried out only in relatively large time intervals and the incremental position determination is used in between.